Primary-beam Diffraction Modulated Electron Emission Research Articles

Oxygen-dosage effect on the structure and composition of ultrathin NiO layers reactively grown on Ag(001)

NiO ultrathin films have been prepared under UHV conditions on Ag(001) substrate by metal deposition in ${\mathrm{O}}_{2}$ atmosphere. The films were prepared by deposition of the amount of metal correspondent to 1 ML (monolayer) of NiO in the presence of different oxygen-to-metal flux ratios, to investigate the oxygen dosage effect on the structure and composition of the growing layer and on the stoichiometry of Ni oxide. Thicker films (up to 20 ML) were also prepared. The structure has been monitored both in reciprocal and direct space by low-energy electron diffraction and primary-beam diffraction-modulated electron emission. Core-level x-ray photoemission spectroscopy has been used to study film chemistry and composition. At low coverage (below 2 ML) a dramatic dependence of structure and composition on the oxygen-to-nickel flux ratio has been observed. Low oxygen dosage induces a $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction in the 1-ML films that evolves to a $(1\ifmmode\times\else\texttimes\fi{}1)$ phase as the dosage and/or the film thickness increases. In the layers prepared at low oxygen dosage a large fraction of metallic Ni coexists with the Ni oxide, but the oxidized fraction largely prevails for high oxygen dosage. The oxygen dosage during the growth also affects thicker films. The mosaic formation, which has been ascribed to misfit strain relaxation, is related to a low oxygen-to-nickel flux ratio.

The Co/Si([formula omitted]) interface formation: a temperature dependent reaction

We have investigated the reaction of Co with the Si(111) surface both at room temperature (RT) and at high temperature (500–650 °C). The temperature evolution of the RT deposited 10 ML film has also been studied. The films, prepared by the different methods, have been structurally characterized by means of primary-beam diffraction modulated electron emission. Auger electron spectroscopy has been used to follow their stoichiometric evolution. For RT deposition the films have been found to have a B-type (180° rotated with respect to the underlying Si(111) surface) cubic structure with a Co content and an interlayer spacing increasing with thickness. After 650 °C annealing, the films are completely reacted and have an unstrained B-type CoSi2 structure. High temperature (500 °C) deposition of Co leads to the formation of stoichiometric CoSi2 films. Both annealed and high temperature grown films are found to be Si terminated.

Structure and growth mode of thin Co films on Fe(001): comparison of purely thermal and ion-assisted deposition

The growth of epitaxial Co layers on Fe(001) by simultaneous thermal evaporation of Co atoms and ion bombardment with low-energy (300–1000 eV) Ar ions has been investigated for a wide range of ion/atom flux ratios (up to 0.5) and compared with growth by a purely thermal deposition procedure. Co nominal thickness was in the 2–50 monolayer (ML) range. At low Co nominal thickness (2–10 monolayers), the thermally grown films tend to form islands, while the ion beam-assisted growth results in the formation of a continuous Co layer. Structural characterisation of the Co layer was performed by exploiting the dependence of the Auger emission intensity on the angle of incidence of the primary exciting electron beam (primary-beam diffraction modulated electron emission, PDMEE). For purely thermal deposition, transition from the cubic phase to the equilibrium h.c.p. phase occurs at a critical nominal thickness of 15 ML, both structures showing a significant strain (4% contraction and 6% expansion compared to the ideal b.c.c. and h.c.p. phases, respectively). Ion assistance has proved to be effective in lowering both the critical thickness of the cubic phase and strain in the hexagonal Co film. The relevant parameter which comes into play in determining the effectiveness of ion assistance has been found to be the average energy deposited per atom.

Breakdown of the bi-dimensional symmetry in bct Fe layers by epitaxy on Co([formula omitted]) surface

The structure of thin Fe overlayers on the Co(112̄0) surface has been investigated by low energy electron diffraction (LEED) and by primary-beam diffraction modulated electron emission (PDMEE), as a function of the Fe coverage (6–50 ML) and of the substrate temperature (between room temperature and 300°C). The structure of the layers was the tetragonally distorted bct modification of the equilibrium bcc phase, with the (001)Fe||(112̄0)Co and [11̄0]Fe||[11̄00]Co relationships with the Co substrate. The Fe(001)–Co(112̄0) system exhibits a very large misfit asymmetry in the interface plane. As a consequence, a breakdown of the long-range order occurs in the Fe film along the [11̄00] substrate direction.

Fe epitaxial layers on Cu 3Au(0 0 1): a structural study by primary-beam diffraction modulated electron emission

The structure of Fe films grown on the (0 0 1) surface of a Cu 3Au single crystal at room temperature, has been investigated by means of primary-beam diffraction modulated electron emission (PDMEE), as a function of thickness in the 0–37 monolayers (ML) range. A pseudomorphic, face centered cubic (fcc) structure has been found for films thinner than 4 ML; in an intermediate region of thickness, which extends up to ∼23 ML, two strained phases, with fcc and body centered tetragonal (bct) structure, are present. For thicker films a unique phase of bct type is detected within the chosen information depth of the PDMEE technique. At 37 ML the surface layers of the film show an unstrained body centered cubic (0 0 1) structure with the [1 0 0] crystal direction parallel to the [1 1 0] direction of the substrate. Segregation and intermixing of substrate atoms with the deposit has also been considered. A fraction of the order of one-tenth of ML of Au atoms has been found to segregate on top of films thinner than 4 ML. Au atoms in a concentration of a few percent have been found to be intermixed within the Fe film. In the low coverage region PDMEE data of the Fe/Cu(0 0 1) system have also been acquired for comparison with those obtained on Fe/Cu 3Au(0 0 1).

Structural characterisation of Fe layers on Co(112̄0)

The structure of thin Fe overlayers on the Co(112̄0) surface has been investigated by low energy electron diffraction (LEED) and by primary-beam diffraction modulated electron emission (PDMEE). Films were prepared by slow Fe deposition at room temperature, up to a nominal coverage of 14 ML. The structure of the layers was the tetragonally distorted bct modification of the equilibrium bcc phase, with the (001)Fe||(112̄0)Co and [11̄0]Fe||[11̄00]Co relationships with the Co substrate. In the low coverage range (0–2 ML) iron forms 3 ML thick islands, that by further Fe deposition increase in thickness and finally coalesce at a coverage as large as 5 ML. The occurrence of a highly distorted bct phase is detected at a nominal coverage of about 2 ML. The final, 14 ML thick film exhibits a 3% contraction along the growth axis with respect to the equilibrium phase. The Fe(001)–Co(112̄0) system is a rarely encountered example in which the substrate and overlayer unit meshes exhibit a very large misfit asymmetry in the interface plane. As a consequence, a breakdown of the long-range order occurs in the Fe film along the [11̄00] substrate direction.

Structural analysis of epitaxial Fe films on Ni(001)

The atomic geometry and growth mechanism of Fe films (0.5–20 monolayers (ML)) epitaxially grown on Ni(001) have been investigated by a multitechnique approach with the twofold aim of assessing the reliability of quick structural techniques (primary-beam diffraction modulated electron emission (PDMEE), secondary electron imaging (SEI)), suitable for on-line monitoring of film growth, and studying the structural evolution of the Fe/Ni system. To this end, samples were analysed by PDMEE and SEI together with synchrotron radiation photoelectron diffraction (PD), and extended x-ray absorption fine structure (EXAFS). Results show the effectiveness of the approach by quick techniques. In the early stage of growth, Fe arranges in a strained fcc(001) structure. Transition to the bcc phase occurs through nucleation of bcc(110) domains with the bcc<111>// fcc<110>orientation. Quantitative analysis based on PD and EXAFS data is also presented.

Structural and electronic properties of thin Co films on Fe(001) and Fe(001)-p(1×1)O in the bct-to-hcp transition regime

We present a combined study of structural and electronic properties of thin Co films (up to 50 ML) grown on Fe(001). By means of primary-beam diffraction modulated electron emission, we found a structural transition between a strained bcc structure (bct) and the stable hcp phase occurring at ∼15 ML Co coverage. This structural change has a counterpart in the unoccupied electron states of the Co overlayer, as seen by absorbed current spectroscopy. We demonstrate that the evolution of AC spectra can be used to monitor the mentioned transition and apply this technique to the case of Co grown on Fe(001)-p(1×1)O. It is found that the presence of the oxygen causes a delayed onset of the bct–hcp transition.

GROWTH MODE OF ULTRATHIN Co FILMS ON Fe(001) PREPARED BY LOW ENERGY ION-ASSISTED DEPOSITION

Co films were epitaxially grown on Fe(001) by simultaneous thermal evaporation of Co atoms and ion bombardment with low-energy (300–1000 eV) Ar ions in a wide range of ion-to-atom flux ratio (0.02–0.5). The 0–50 ML coverage range was investigated. Transition from island growth to a continuous layer growth occurs on passing from purely thermal to ion-assisted deposition procedure. Structural characterization was performed by Primary-beam Diffraction Modulated Electron Emission (PDMEE). For purely thermal deposition, transition from the bcc phase to the equilibrium, hcp phase has been observed at a critical coverage of about 15 ML, both structures showing a significant strain (7% contraction and 5% expansion with respect to the ideal bcc and hcp phase, respectively). Ion assistance has proved to be effective in lowering both the strains in the Co film and the critical thickness of the bcc phase.

Structural and compositional stability of Co oxide grown on (001) bct Co

Room temperature (RT) adsorption of oxygen on the (001) surface of metastable, cubic Co film epitaxially grown on a Fe substrate results in the formation of about 7 monolayers (ML) crystalline CoO film, in the typical rocksalt structure 45° rotated with respect to the square unit mesh of the underlying Co film. We investigated the stability of this oxide layer upon annealing in the RT–620 K temperature range, by means of primary-beam diffraction modulated electron emission (PDMEE), and X-ray photoelectron spectroscopy (XPS). For temperatures up to 570 K, film thickness and local order are preserved, in spite of an increasing number of Co and O atoms bonded in sites of reduced coordination. For larger temperatures, thickness also reduces, but the oxide film is still characterized by a high degree of CoO-like local order.

Growth and structure of cobalt oxide on (001) bct cobalt film

The interaction of oxygen with the (001) surface of a metastable cubic cobalt film, 10 monolayers (ML) thick, epitaxially grown on the Fe(001) surface, has been studied at 300 and 470 K by means of primary-beam diffraction modulated electron emission, X-ray photoelectron spectroscopy and Auger electron spectroscopy. At 300 K the formation of 7 ML of CoO has been observed, in the typical rocksalt structure, rotated by 45° with respect to the square unit cell of the unreacted, underlying cobalt film. A relevant fraction of cobalt and oxygen atoms with reduced coordination has been detected which does not affect significantly the local order in the oxide film. At 470 K, the oxide extends in thickness involving almost the full cobalt film. For both room-temperature and high-temperature growth conditions, the resulting oxide layer is tetragonally distorted.

Epitaxial growth of ultrathin Fe films on Ni(001): a structural study

We have investigated the atomic geometry of Fe films on a Ni(001) crystal in the thickness range 0–25 ML with the aim of following the transition from the fcc pseudomorphic structure to the equilibrium bcc phase. The structural techniques used are primary-beam diffraction modulated electron emission (PDMEE) and low-energy electron diffraction (LEED). For film thicknesses up to 5 ML the films are fcc, strained in the direction of film growth. For higher Fe coverages, the films begin a transition to the bulk bcc structure which is complete after 14 ML of Fe coverage. We have obtained evidence for intermixing between Fe and Ni in the first 3 ML. The orientational relationship between the bcc phase and the underlying fcc substrate has also been investigated, finding a bcc (110) phase with four possible domains, each with one of the 〈111〉 directions parallel to an fcc 〈110〉 direction.

Modulated electron emission for structural characterization of buried layers and interfaces

Principles and performances of Primary-beam Diffraction Modulated Electron Emission are briefly reviewed. This approach exploits the spatial modulation of the beam electrons waves intensity in ordered atomic arrays, which in turn results in a marked dependence of the electron emission on the incidence angle. Detection of Auger electrons or inelastically backscattered electrons (ionization losses) provides chemical specificity. Surface sensitivity is related to the inelastic mean free path of the detected electrons. Examples are given of the capability of this technique for structural characterization of buried layer and interfaces. They include: i) epitaxial growth of Co oxide on the (001) surface of Co bct film, ii) subsurface behaviour of Co epitaxial layers during the bct→hcp phase transition.

Epitaxial growth of CoO on the (001) surface of bct cobalt

Adsorption of oxygen on a (001) surface of 10 monolayers thick, metastable cubic Co film epitaxially grown on the Fe(001) surface was studied at 300 and 470 K by means of primary-beam diffraction modulated electron emission, X-ray photoelectron spectroscopy and Auger electron spectroscopy. At 300 K, the formation of CoO in the very first layer was observed, in the typical rock salt structure, 45° rotated with respect to the square unit mesh of the remaining, underlying Co film. At 470 K, the oxide extends in thickness involving the whole Co film. Similar behavior has been observed for the (11 2 ̄ 0) surface of the hcp phase. On both surfaces, the oxide layer appears tetragonally distorted.

SURFACE AND NEAR SURFACE STRUCTURE OF Fe–Co LAYERS BY SCATTERING-INTERFERENCE OF PRIMARY ELECTRONS

We studied the Co/Fe bi- and multilayer growth on Fe single crystal using the primary-beam diffraction modulated electron emission (PDMEE) technique. This approach enables a chemical selected structural characterization of both surface and buried layers and interfaces to be performed. Co growth on the Fe(001) surface was studied in the 3–70 ML coverage range. The transition from the initial epitaxial bcc phase of the Co film to the stable hcp phase was found to occur rather abruptly at 25–30 ML coverage, a value larger than previously reported in the literature. Subsequent Fe deposition on the Co hcp film recovers the bcc structure in the sandwiched Co layer. However, removal of the Fe epilayer by mild sputtering results again in an hcp Co film.

Early stage in low energy ion-induced damage on InP(110) surface

The change in the short-range order created by ion milling in the near surface region of InP single crystals was investigated by primary beam diffraction modulated electron emission (PDMEE). The very early stage of the damage creation by low energy (0.6–1 keV) Ar ions in normal and oblique incidence was studied. A simple model based on the weighted combination of perfectly crystalline and completely amorphous regions was used to model the experimental results. Evidence of a subsurface nucleation of the amorphization process was found. We also found that the total sputtering yield is markedly dependent on the ion dose, being on the undamaged surface much larger than its steady state value. Low energy electron diffraction (LEED) measurements were also performed to correlate long-range and short-range order removal by ion bombardment. Finally, the ion damage on the GaAs and InP surfaces was comparatively discussed.

Element-specific, surface and subsurface structural analysis by scattering-interference of primary electrons

We investigated the effects of scattering-interference of primary electrons on the secondary electron emission intensity fron ordered surfaces and interfaces. Because of the focusing-defocusing of the primary wave along atomic chains, maxima in the electron yield occur when the exciting beam is aligned with low index axes. Therefore the electron intensity distributions as a function of the angle of incidence of the primary beam can be interpreted as projected images of real space, local atomic arrangement. The process can be modelled in a single scattering cluster (SSC) approximation. Potential of primary-beam diffraction modulated electron emission (PDMEE) technique for surface and subsurface structural characterization is shown, for Co epitaxy on Fe(001).

Substrate amorphization induced by the sputter deposition process: Geometrical aspects

Sputter deposition of thin films and multilayers involves, in the initial stage, the energetic interaction of reflected ions and/or sputter ejected target particles with the substrate surface and with the overlayer-substrate interface.The present work reports on the c-Si(111) surface amorphization when Ge is deposited by 4 keV Ar+ ion sputtering in different experimental geometries. The incidence angle of the Ar+ beam was 65°, with respect to the surface normal, to emphasize prompt collision sputtering. Attempts have been made to clarify the role of the different particles involved in the process. New, unconventional approaches to Auger spectroscopy were used for this study. Substrate structural characterization during overlayer growth has been performed by primary-beam diffraction modulated electron emission (PDMEE) technique, a powerful tool for the investigation of ‘‘buried’’ surfaces. Auger electron kinetic emission from the growing interface was used to determine the occurrence and importance of the different collisional processes. The high average kinetic energy of the Ge atoms ejected from the target in prompt, first-order collisions was found to be responsible for the large substrate amorphization we observed with a specular geometry but not on off-specular geometry. Reflected, energetic Ar+ ions do not play a relevant role.

Primary‐beam diffraction modulated electron emission (PDMEE) for surface structural analysis

AbstractScattering interference of primary exciting electrons leads to a dependence of the Auger emission intensity on the beam incidence angle. For beam energy in the kiloelectron‐volt range, maxima in the Auger yield occur when the primary beam is aligned with low‐index crystal axes, therefore the Auger angular intensity distribution directly reflects the symmetries and the degree of local order. We investigated the potential for this effect, primary‐beam diffraction modulated electron emission (PDMEE) to be used for surface structural characterization, in comparison with the Auger electron/photoelectron diffraction (AED/PD) technique. The angular intensity distributions of both low and high kinetic energy Auger electrons were measured on both GaAs and InP (110) surfaces. We show, by using highly collimated energetic electron beams, that the angular resolution is significantly improved in PDMEE with respect to AED/PD. Unlike the AED/PD case, the angular intensity distribution is independent of the kinetic energy of the Auger electrons, therefore low‐energy, surface‐sensitive electrons also directly reflect crystal symmetries, owing also to the fact that the specific momentum character of the Auger electrons can be neglected. Anisotropy in the intensity angular distribution as large as 70% was measured, comparable to AED/PD anisotropy values. Both a retarding field (low‐energy electron diffraction optics) and a dispersion field cylindrical mirror analyser were used for PDMEE analysis, and instrumental performances are discussed comparatively.